sort: add is_sorted (#1660)

* sort: add is_sorted
Add is_sorted function to check whether a list is sorted or not. Sort
order (ascending or descending) will be detected by looking at the data.
Add tests.
* update the release notes
* refactor: use lambda

lib/std/sort/sorted.c3

@@ -0,0 +1,59 @@
+module std::sort;
+
+<*
+Returns true if list is sorted in either ascending or descending order.
+ @require @is_sortable(list) "The list must be indexable and support .len or .len()"
+ @require @is_valid_cmp_fn(cmp, list, ctx) "Expected a comparison function which compares values"
+ @require @is_valid_context(cmp, ctx) "Expected a valid context"
+*>
+macro bool is_sorted(list, cmp = EMPTY_MACRO_SLOT, ctx = EMPTY_MACRO_SLOT) @builtin
+{
+	var $Type = $typeof(list);
+	usz len = sort::@len_from_list(list);
+	if (len <= 1) return true;
+	var check_sort = fn bool($Type list, usz start, usz end, $typeof(cmp) cmp, $typeof(ctx) ctx)
+	{
+		usz i;
+		int sort_order;
+
+		// determine sort order (ascending or descending)
+		for (i = start; i < end && sort_order == 0; i++)
+		{
+			sort_order = @sort_cmp(list, i, cmp, ctx);
+		}
+
+		// no sort order found, all elements are the same, consider list sorted
+		if (sort_order == 0) return true;
+
+		// compare adjacent elements to the sort order
+		for (; i < end; i++)
+		{
+			if (sort_order * @sort_cmp(list, i, cmp, ctx) < 0) return false;
+		}
+		return true;
+	};
+	return check_sort(list, 0, len - 1, cmp, ctx);
+}
+
+macro int @sort_cmp(list, pos, cmp, ctx) @local
+{
+	var $has_cmp = @is_valid_macro_slot(cmp);
+	var $has_context = @is_valid_macro_slot(ctx);
+	var $cmp_by_value = $has_cmp &&& $assignable(list[0], $typefrom($typeof(cmp).paramsof[0].type));
+
+	var a = list[pos];
+	var b = list[pos+1];
+
+	$switch
+		$case $cmp_by_value && $has_context:
+			return cmp(a, b);
+		$case $cmp_by_value:
+			return cmp(a, b);
+		$case $has_cmp && $has_context:
+			return cmp(&a, &b, ctx);
+		$case $has_cmp:
+			return cmp(&a, &b);
+		$default:
+			return compare_to(a,b);
+	$endswitch
+}